Tumor infiltrating lymphocytes are currently under investigation in Surgery Branch clinical protocols for the adoptive immunotherapy-of patients with advanced cancers. Responses to therapy have been observed in select patients with metastatic melanoma. Elucidating the mechanisms by which TIL recognize and destroy tumor cells is essential to optimizing clinical protocols. The following are areas under study: 1.Mechanisms of tumor resistance to TIL killing. A melanoma tumor line resistant to lysis by autologous TIL was immunoselected from a TIL"sensitive melanoma. The variant appeared to have lost a cell surface determinant critical to TIL recognition. Experiments to define this determinant are in progress. 2.Nature of the melanoma tumor antigen recognized by TIL. TIL lysis of panels of HLA-matched melanomas revealed that multiple shared and possibly unique tumor antigens exist which can be recognized by TIL. 3.Nature of TIL response to tumor. In addition to direct lysis of tumor targets, some TIL are capable of producing a variety of cytokines upon specific stimulation with autologous tumor, but not with allogeneic tumors. This response has been documented in TIL derived from 3 of 4 melanomas, and 1/7 breast carcinomas investigated. Studies of colon carcinoma are in progress. Cell separation studies show that CD8+ TIL are responsible for cytokine production as well as lysis.